1. Field of the Invention
This invention relates generally to apparatus for locating mechanical defects in a specimen having an extensile direction, such as a wire rope; and more particularly to an improved tester that is relatively insensitive to uniform characteristics such as the material and lay of a wire rope--and accordingly is relatively more sensitive to small defects.
2. Prior Art
Wire rope is used in myriad industrial applications--notably including control cables in aircraft and support cables for elevators, hanging scaffolds and cranes, and mining equipment. In many, if not the great majority, of such applications, lives as well as costly equipment depend immediately upon the integrity of the wire rope.
Until about a dozen years ago, however, the only commercially available testing method for such relatively small-diameter wire rope was primitive. It consisted of a manual "rag wipe" of the rope to feel for broken wires and loose wire bundles ("bird caging"), supplemented by a close visual inspection for corrosion.
Such a method fails to detect many internal defects such as internal breakage, bird caging, corrosion, wear, and work hardening. In fact, even external wear and corrosion are often overlooked in such testing. Nevertheless this method is still commonly used throughout industry.
Electromagnetic testing of wire rope has been attempted since the early 1800's, using several different sensor arrangements. As far as I know, these devices were all directed to inspecting rope of relatively large diameter--one to three inches--used primarily in mines. The sensitivity of these devices was relatively coarse.
In 1975, however, I introduced a wire-rope tester that operates on an electromagnetic principle. It consists of a field-developing coil, a split sensor coil, and electronic circuitry for energizing the field coil and detecting fluctuations in the current from the sensor coil.
The sensor coil, though it has a gap in which a segment of the wire rope is positioned, is wound as a single coil. Its two sections are conaxial with each other and with the field coil.
The electronics in my prior tester includes a "tuned tank" circuit, in which the uniform component of the cable reluctance contributes to the tank frequency. Therefore the testing instrument is preliminarily tuned for each new wire-rope specimen.
That testing instrument has been adopted by the U.S. Air Force, Coast Guard, and Navy--and many foreign military services as well--as the required method of control-cable inspection on the C-130 aircraft and others. Accordingly the instrument has saved the lives of innumerable crew members and passengers.
That instrument does, however, have certain limitations. First, because of the sensor arrangement and type of circuitry employed, the largest rope that can be inspected is approximately 5/32 inch in diameter.
Secondly, when a wire rope is passed through the probe, the periodic structure or "lay" of the rope causes pulses in the detection circuit. Such pulses are--but for their periodicity--indistinguishable from those caused by small localized defects. This effect limits the sensitivity of the instrument to such defects.
The prior art consequently leaves room for improvement in the field of wire-rope testing.